1. Field of the Invention
The present invention relates to an input pen for a coordinate input apparatus for determining an input position on the basis of a vibration propagating on a vibration transmitting plate.
2. Description of the Related Art
Conventionally known coordinate input apparatuses detect an input elastic wave vibration from an input pen by a plurality of sensors and thereby detect coordinates indicated by the input pen. For example, the pen tip of an input pen for a coordinate input apparatus described in Japanese Patent Laid-Open No. 1-114924 is constructed of polyamidoimide as a resin and:
(1) has high resistance to wear caused by, e.g., friction when coordinates are input; and
(2) can efficiently transmit vibrations generated inside the input pen without any decay.
Since the pen tip of the input pen of the above conventional coordinate input apparatus is made of polyamidoimide as a resin, it is expected that mass-production by molding is possible. However, the molded pen tip of the input pen has the following problems.
When the input pen incorporating the molded pen tip vertically abuts against a vibration transmitting plate and the input pen is rotated around its axis, a signal waveform detected by sensors changes. That is, when an input vibration from the input pen concentrically propagates in the form of a wave from the input point on the vibration transmitting plate, the detected signal waveform changes in accordance with the direction of the input pen. This phenomenon is called directivity and has a harmful influence as follows.
The basic principle of the conventional coordinate input apparatus is to calculate a distance by using the transmission time and ultrasonic velocity of acoustic wave. It is desirable that not only the ultrasonic wave be constant in a propagating body and the signal waveform to be detected by sensors be always the same. That is, as shown in FIG. 7, even when a vibration is input from a single point, detected propagation times are different if different signal waveforms are detected. Referring to FIG. 7, a vibration is input from a single point, so the same value is supposed to be detected at propagation delay time 1 and propagation delay time 2. However, if the waveform changes for some reason, different values are detected. As a consequence, the coordinate input apparatus causes detection error, i.e., the apparatus operates as if input vibrations from different points were detected. This decreases the accuracy of the coordinate input apparatus. To realize a reliable coordinate input apparatus, therefore, a construction always capable of detecting the same signal waveform is necessary.
As described above, the directivity of an input pen decreases the coordinate calculation accuracy of a coordinate input apparatus of this sort. To realize a coordinate input apparatus with high accuracy and high reliability, some countermeasure is necessary.
Additionally, the ability to transmit vibrations is an essential specification of an input pen of the above sort, so a resin as a relatively hard material is chosen. However, this property of being xe2x80x9chardxe2x80x9d has the following harmful influence from the user""s viewpoint. That is, a coordinate input surface transmits input vibrations from an input pen to vibration detecting elements, so glass or a metal such as aluminum is chosen as the material. Accordingly, when a user inputs coordinates by using this input pen, the input pen makes the user feel a kind of xe2x80x9chardnessxe2x80x9d, and this degrades the operability during coordinate input. For example, a user easily becomes tired when he or she uses the input pen for hours.
Another problem is that the pen tip of an input pen of a coordinate input apparatus of this sort constantly wears when an operator writes with the pen. Although the durability can be improved by the use of a resin with high wear resistance, the pen tip wears anyway after long use. Therefore, the pen tip is desirably replaceable. A construction as disclosed in Japanese Patent Laid-Open No. 06-119102 is one such prior art. The problem of this construction is the loss of ultrasonic wave transmitted from a vibration transmitting member to a pen tip. That is, in this prior art, the energy of passing ultrasonic wave can be held constant only when the screw fastening torque (this relates to the contact force between the vibration transmitting member and the contact surface of the pen tip) is controlled. Strictly speaking, if no satisfactory screw fastening torque is obtained, vibrations cannot be transmitted to the pen tip any longer, so no coordinates can be input. Specifications requiring a user to control the screw fastening torque decrease the degree of customer""s satisfaction. To solve this problem, a pen tip has been proposed by which the contact force between the vibration transmitting member and the contact surface of the pen tip is held constant even when a user replaces one pen tip with another. However, this prior art requires a construction always capable of obtaining stable vibrations even when a user replaces one pen tip with another, resulting in high manufacturing cost.
It is an object of the present invention to provide an input pen for a coordinate input apparatus, which can improve the operability and reduce the directivity of the input pen and can be readily replaced with another.
To achieve the above object, an input pen according to the present invention has the following constructions.
An input pen for a coordinate input apparatus for determining an input position on the basis of a vibration propagating on a vibration transmitting plate, comprises a vibration generating element for generating a vibration, a vibration transmitting member for transmitting the vibration generated by the vibration generating element, and a pen tip member attached to a point of the vibration transmitting member and made of a material having viscoelasticity.
The material having viscoelasticity is preferably acrylonitrilebutadiene rubber.
The material having viscoelasticity is preferably one material selected from the group consisting of rubber materials such as chloroprene rubber and butyl rubber, elastomers such as styrene-, olefin-, urethane-, polyester-, polyamide-, and vinyl chloride-based elastomers and a combination thereof.
Preferably, the vibration transmitting member has a semispherical point, the pen tip member has a semispherical cavity bottom, and a radius of the cavity bottom is smaller than a radius of the point.
The surface of the pen tip member is preferably so treated as to decrease frictional resistance against the vibration transmitting plate.
The thickness of the pen tip member is preferably 0.5 to 2 mm.
Preferably, the vibration transmitting member has a plane point, the pen tip member has a plane cavity bottom, and a radius of the cavity bottom is smaller than a radius of the point.
Preferably, the vibration transmitting member has a spherical point, the pen tip member has a spherical cavity bottom, and a radius of the cavity bottom is smaller than a radius of the point.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.